Fix build break when VS2015 is not installed.

[vancem]

If you ONLY install VS2017 (now the common scneario)
and build coreclr, it will fail with COM with a Class_Not_Registered error

This is because a build-tools package is depending on an old COM object
that has gone way in VS2017.

While we sort getting a proper fix, this makes the work-around (which
is registering the DLL by hand) discoverable.

See https://github.com/dotnet/coreclr/issues/11305 for details.

@lt72 @russellhadley @briansull

[vancem]

@dotnet-bot test Ubuntu armlb Cross Debug Innerloop Build

[vancem]

@dotnet-bot test Perf Build and Test

[briansull]

I had to use a 32-bit command prompt to register this:
(Using a 64-bit command prompt looked likes it did something, but it still wasn't registered where it needed to be)

Use the following to start a 32-bit command window:

%SystemRoot%\sysWow64\cmd.exe

[vancem]

Hello Brian,

You had mentioned when we talked that you need to run regsvr32 in a 32 bit command window (so you get the 32 bit version of regsvr32). My experimentation suggested that this is not true (when regsvr32 is applied to a 32 bit DLL, it registers it as a 32 bit COM object).

I will see if I can reproduce this on a 'clean' system to confirm.

[vancem]

@briansull I just tried an experiment on a clean system where I ran dactableGen and saw the failure, then did a regsvr32 (in a 64 bit window), on msdia120.dll and afterward it worked properly.

I think we can stick with the current instructions. If find they are wrong we can correct them